Fuel vapor extraction system

ABSTRACT

A fuel vapour extraction system is described for an internal combustion engine ( 10 ) that is supplied with a volatile liquid fuel from a fuel storage tank ( 20 ). The engine ( 10 ) has an air intake system ( 12, 14, 16 ) and a liquid fuel injection system for dispensing fuel to mix with air to be burnt in the engine. The fuel vapour extraction system includes a volatizing chamber ( 30 ) connected to the fuel storage tank ( 20 ) by a valve ( 26 ) serving to maintain a constant liquid level of fuel in the chamber ( 30 ). A pipe ( 42 ) connected to one or more vacuum sources leads into the vapour space above the liquid level in the chamber ( 30 ) and maintains a reduced pressure in the volatizing chamber. The fuel injection system includes a fuel circulation pump ( 32 ) for drawing liquid fuel from the volatizing chamber ( 30 ) and supplying the fuel under pressure to a fuel rail ( 34 ), fuel injectors ( 18 ) for dispensing metered quantities of fuel from the fuel rail ( 34 ) to the engine cylinders, and a relief valve ( 36 ) for maintaining a constant fuel pressure in the fuel rail ( 34 ) and returning unused fuel from the fuel rail ( 34 ) to the reduced pressure vapour space in the volatizing chamber ( 30 ) by way of a fuel return pipe ( 38 ). An evaporator ( 40 ) is provided in the vapour space of the volatizing chamber ( 30 ) to act as a means for increasing the surface area to volume ratio of the return fuel. The return fuel comes into intimate thermal contact with the evaporator ( 40 ) and spreads over a large area of the evaporator ( 40 ) exposed to the reduced pressure in the volatizing chamber ( 30 ).

FIELD OF THE INVENTION

The present invention relates to a fuel vapour extraction system for aninternal combustion engine supplied with a volatile liquid fuel from afuel storage tank, the engine having an air intake system and a liquidfuel injection system for dispensing fuel to mix with air to be burnt inthe engine.

BACKGROUND OF THE INVENTION

There are known, in particular from marine applications, fuel vapourextraction systems for an internal combustion engine that is suppliedwith a volatile liquid fuel from a fuel storage tank and that has an airintake system and a liquid fuel injection system for dispensing fuel tomix with air to be burnt in the engine. The fuel vapour extractionsystem includes a volatising chamber connected to the fuel storage tankby a valve serving to maintain a constant liquid level of fuel in thechamber and a vapour space above the liquid level in the chamber, andmeans for drawing vapour from the vapour space in order to maintain areduced pressure in the volatising chamber. The fuel injection systemincludes a fuel circulation pump for drawing liquid fuel from thevolatising chamber and supplying the fuel under pressure to a fuel rail,fuel injectors for dispensing metered quantities of fuel from the fuelrail to the engine cylinders, a relief valve for maintaining a constantfuel pressure in the fuel rail and a fuel return pipe for returningunused fuel from the fuel rail to the volatising chamber. Examples ofsuch systems are to be found in U.S. Pat. Nos. 5,647,331, 5,115,784 and5,579,740 and in WO89/06312.

Such systems are employed in marine applications because safetyregulations relating to marine vessels in some countries forbid thereturn of fuel from the injection system to the main fuel storage tank.Instead the fuel is returned to a separate chamber and steps are takento extract vapour from the latter chamber to avoid vapour lock in thefuel. These systems do not intentionally fraction the fuel to enable theengine management system to make the best use of the differentfractions.

U.S. Pat. No. 5,373,825 discloses a fuel vapour extraction systemintended for an engine burning a heavy oil that comprises a volatisingchamber separate from the fuel tank for intentionally volatising thelighter fraction of the oil. Within the volatising chamber, the oil isheated by a heating element and the lighter fraction of the oil isdriven out by the applied heat at substantially ambient air pressure,ambient air being admitted into the chamber to mix with the gasifiedfuel-and transport it to the air supply of the engine. The remainingliquid fraction that is not gasified is also drawn from the volatisingchamber by a fuel injection system and injected into the combustionchamber of the engine. Only a small quantity of surplus oil, that is notinjected into the engine, is recycled to the volatising chamber.

This vapour extraction system has the advantage of achieving acontinuous supply of fuel vapour, the availability of which can be usedto advantage by a suitably designed engine operating with vaporisedfuel. However, to achieve a continuous vapour supply, heating energymust be applied continuously to heat the oil which is a drain on thefuel consumption of the engine. The heating element raises thetemperature of the oil to a point where the lighter fraction begins toboil and further heating then provides the latent of heat ofvaporisation for maintaining a steady gasifying rate.

If the invention is applied to gasoline fuel instead of a heavy oil, theremaining liquid fraction that is drawn from the volatising chamber willbe too hot and will need to be cooled before it is delivered to the fuelinjection system in order to avoid vapour lock in the fuel injectionsystem. If the bulk of the fuel is recirculated, as occurs in gasolineengines under idle and low load conditions, the system becomes verywasteful of energy as the same fuel is repeatedly heated and thencooled, which is reflected in high fuel consumption.

There are several other disadvantages associated with the application ofexternal heat to the fuel in the volatising chamber. For example, duringcold start, there will not be available an adequate supply of vapouruntil the temperature of the fuel has been raised sufficiently.Furthermore, because of the slow response of the heating element, itwill not be possible to increase the vapour flow rapidly when there issudden increase in the demand for vapour. Also, because of the limitedrate at which the heated fuel can be cooled, it will not then bepossible to cool the increased flow of the hot liquid fuel sufficientlyrapidly before it enters the fuel injection system, thereby riskingvapour lock.

OBJECT OF THE INVENTION

The present invention therefore seeks to provide a continuous supply offuel vapour that does not rely on the use of an externally poweredheating element to heat the fuel to promote its vaporisation.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a fuel vapourextraction system for an internal combustion engine supplied with avolatile liquid fuel from a fuel storage tank, the engine having an airintake system and a liquid fuel injection system for dispensing fuel tomix with air to be burnt in the engine, the fuel vapour extractionsystem including a volatising chamber connected to the fuel storage tankby a valve serving to maintain a constant liquid level of fuel in thechamber and a vapour space above the liquid level in the chamber, andmeans for drawing vapour from the vapour space in order to maintain areduced pressure in the volatising chamber, the fuel injection systemincluding a fuel circulation pump for drawing liquid fuel from thevolatising chamber and supplying the fuel under pressure to a fuel rail,fuel injectors for dispensing metered quantities of fuel from the fuelrail to the engine cylinders, a relief valve for maintaining a constantfuel pressure in the fuel rail and a fuel return pipe for returningunused fuel from the fuel rail to the volatising chamber, characterisedby means within the vapour space of the volatising chamber for promotingevaporation of the return fuel by significantly increasing the surfacearea to volume ratio of the return fuel.

The means for promoting evaporation can be means for atomising the fuelinto fine droplets or the fuel may be allowed to fall on an evaporatorof large surface area.

In use, fuel evaporates off the evaporator or from the droplets byvirtue of the reduced pressure biasing the vapour equilibrium in thevolatising chamber, and in the process cools the remaining fuel bydrawing heat from it to supply the latent heat of vaporisation. Thisheat is replenished by transfer of heat contained in the fuel returningto the volatising chamber after circulating through the fuel rail andpicking up waste heat from the fuel pump, the engine block and theengine compartment.

Although the transfer of heat from the fuel is relatively small for eachpass of the fuel through the volatising chamber, sufficient heat canstill be extracted to support the vapour equilibrium and maintain asteady evaporation rate if the fuel is circulated at a high flow rate.

The volatile liquid fuel in the fuel storage tank may be a singlecomponent fuel like methanol, or it may be a blend of hydrocarbon fuelslike gasoline having a range of boiling points.

The means for drawing the fuel vapour and maintaining a reduced pressurein the vapour space of the volatising chamber may be a venturi sectionin the air intake passage leading to the intake system of the engine, orit may be a low pressure region in the intake system of the enginedownstream of the engine main throttle. Alternatively these means may bea vacuum pump driven directly or indirectly by the engine.

In contrast with the prior art discussed earlier, the present inventionrelies on controlling the vapour equilibrium in the volatising chamberto regulate the availability of the vapour. This process is reversibleand the exchange of vapour with the liquid may be in either directiondepending on the applied pressure in the volatising chamber, vapourcoming out of the liquid if the pressure in the vapour space is furtherreduced while vapour going back into the liquid if the pressure isincreased. This is accompanied by cooling or heating of the fuelrespectively which in turn would reduce the rate of exchange of vapour.In order to prevent this and achieve a steady rate of vapour exchange, astable temperature in the evaporator must be maintained and this isachieved by passing the recirculated fuel over the evaporator toreplenish the heat loss or heat gain by exchanging heat between the fueland the evaporator.

In the present invention, although a reduced pressure is only applied tothe volatising chamber for the purpose of extracting fuel vapour fromthe volatising chamber, the above vapour exchange in either directioncan still occur under dynamic conditions when the reduced pressure isvaried from a lesser vacuum to a higher vacuum or vice-versa. The timeconstant for the equilibrium to stabilise would depend on the volumecapacity of the vapour in the volatising chamber. This time constant maybe extended by increasing the vapour storage within the volatisingchamber.

It is preferred for the evaporator to be a matrix of fine capillarytubes or porous granules. To extend the equilibrium time constant, thematerial of the matrix may be chemically active to act as a vapourstore, for example, it may be formed of activated carbon.

In the invention, at no time is the liquid fuel anywhere in the fuelsystem hotter than the normal temperature at which the fuel injectionsystem is designed to operate thus eliminating any risk of vapour lock,yet the amount of heat required to sustain a predetermined evaporationrate will be adequate provided that sufficient flow circulation in thefuel injection system is maintained. Under low engine load conditions,the amount of vapour drawn from the vapour space and the amount of theliquid fuel injected into the engine are both small with the result thatthe circulation flow will be large and will easily meet the requirementto support the desired evaporation rate. Under high load conditions, theengine fuel demand is high and the resulting circulation flow will besmall. As a result, the rate at which vapour is exchanged in equilibriumin the volatising chamber will be reduced but that does not present aproblem as the engine at this time is better operated with little or novapour in order to maximise its output power.

Because of the large exposed area of the evaporator or the fineatomisation of the return fuel, a substantial flow of fuel vapour may beextracted from the volatile liquid purely by reduced pressure biasingthe vapour equilibrium, the vapour flow being a function of theavailable exposed area and the differential vapour pressure, this beingthe difference between the reduced pressure in the vapour space and thesaturation vapour pressure of the liquid at the temperature of theevaporator.

By providing a large surface area from which fuel can evaporate andapplying a predetermined reduced pressure below the saturation vapourpressure, the amount of vapour fuel fraction extracted from theevaporator may be varied from a few percent to 100% by weight of thefuel as long as a stable differential vapour pressure can be maintained.On the other hand, the differential vapour pressure may change duringthe evaporation process because of cooling of the evaporator causing thesaturation vapour pressure to decrease and reducing the evaporation rateas a consequence.

In the present invention, the temperature of the remaining fuel can bekept substantially constant by the heat transported to the volatisingchamber by the flow of fuel circulating in the fuel injection system,counteracting the cooling effect and maintaining a steady evaporationrate according to the applied reduced pressure.

The instantaneous flow rate of vapour fraction leaving the volatisingchamber may be regulated by a metering valve provided that the densityof the vapour upstream of the valve and the pressure drop across thevalve are known, and these may be determined using measurements from apressure sensor and a temperature sensor in the volatising chamber. Forany desired ratio of the fuel fractions to be supplied to the engine invapour and liquid forms, the total fuel quantity may be dividedaccordingly and each fraction metered separately to the engine by way ofthe vapour metering valve and the liquid fuel injectors, respectively.

During idle and part load operations, a substantial proportion of fuelvapour may be introduced into the intake manifold of the engine, whilethe appropriate amount of the remaining liquid fuel to make up theoriginal composition of the volatile fuel may be dispensed separatelyinto the intake ports or directly into the engine cylinders by way ofthe fuel injection system.

During high load operation, the fuel vapour may be shut off and 100% ofthe volatile fuel is dispensed by the liquid fuel injection system.

A range of ratios of the vapour and liquid flow fractions may be drawnfrom the volatising chamber while the operating point in the chambermoves to find its own equilibrium. For example, if the liquid fractionis not transported out of the chamber by not dispensing any fuel throughthe fuel injection system, the equilibrium will move until the vapourwould transport out 100% of the fuel. On the other hand, if the vapourfraction is not transported out of the chamber by shutting off thereduced pressure source, the vapour pressure in the chamber will riseuntil it equals the saturation vapour pressure and the equilibrium willmove in the opposite direction until the liquid would transport out 100%of the fuel. In other words, the system will respond according to demandprovided that sufficient time is allowed for it to reach equilibrium.

During dynamic conditions, the time constant for the operating point toreach equilibrium will be long, but the flow rate of the vapour fractionmay be instantaneously increased by increasing the vacuum and by drawingfrom the vapour store in the evaporator while the flow rate of theliquid fraction delivered to the fuel injection system may beinstantaneously decreased in a balanced manner such that aquasi-equilibrium would still exist in which the overall composition ofthe fuel consumed in the two fractions together remains the same as theoriginal composition of the volatile fuel drawn from the fuel storagetank. As a result, a balance in mass flow can still be maintained evenunder dynamic conditions.

If desired, the balance may be temporarily disturbed for short periodsduring certain engine operations, to achieve additional advantages. Forexample, when starting the engine from cold, an immediate and copioussupply of fuel vapour may be extracted from vapour store in thevolatising chamber. This would improve the cold start quality and lowerthe exhaust emissions during warm up of the engine. The vapour may alsobe pumped into the exhaust system of the engine and burnt upstream of acatalytic convertor to heat the convertor rapidly to its light-offtemperature immediately after a cold start.

In another example, it is known in gasoline fuel that the low boilingpoint hydrocarbons have low octane numbers and the high boiling pointhydrocarbons have high octane number, the two fractions together makingup the average octane number for the complete fuel blend. There istherefore a knock tolerance advantage if the fuel blend is alteredtemporarily to bias towards the high boiling point hydrocarbons at leastduring heavy acceleration modes.

The present invention can be used to achieve this by disproportionatelydecreasing the vapour fraction and increasing the liquid fractiondelivered overall to the engine during heavy acceleration. In this casean increased quantity of the heavy-end of the fuel may be dispensed tothe engine while the vapour store in the evaporator will absorb andretain the associated quantity of the light-end of the fuel, such thatthe total composition of the fuel is fully contained.

If the fuel storage tank is fitted with a vapour storage canister, apurge connection for the vapour canister may be integrated with the fuelvapour extraction system of the present invention. In this case, whenthe vapour canister is purged, the resulting flow of vapour and air maybe arranged to pass through the volatising chamber on its way to theintake system of the engine while maintaining a reduced pressure in thechamber. Some of the purged vapour may condense back into the liquidphase while the air would be fully saturated with vapour.

In the present invention, there is no need at any time to return fuel tothe main fuel storage tank. As a result, the risk of building up oftemperature and pressure in the fuel tank is minimised. This reduces theloading on the vapour storage canister for evaporative emissions controlof the fuel tank and reduces the purge flow necessary to regenerate thecanister during the statutory emission test cycle.

In some known prior art systems, fuel vapour is extracted directly fromthe main fuel storage tank by vacuum and without heating the fuel. Thepresent invention works on a similar principle but has the advantageover such prior art systems that the quality of the fuel in the fueltank does not deteriorate progressively even though vapour iscontinuously being produced. Because the unvaporised liquid fuelfraction is always consumed at a balanced rate with the vapour fuelfraction, the composition of the fuel consumed overall will matchexactly that present in the fuel storage tank and no surplus liquid willbe accumulated.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described further, by way of example, withreference to the accompanying drawing, in which the single FIGURE showsa schematic diagram of an engine with a fuel vapour extraction system ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawing, an engine 10 has an intake manifold 16, a main throttle14 and an intake passage containing a venturi 12. A fuel injectionsystem for the engine comprising a fuel circulation pump 32 thatsupplies fuel under pressure into a fuel rail 34 from which fuel isdispensed to the individual cylinders of the engine by fuel injectors18. The pressure in the fuel rail 34 is regulated by a relief valve 36that derives a reference pressure from the intake manifold 16. Surplusfuel is spilled by the relief valve 36 into a fuel return pipe 38.

While it is conventional for the pump 32 and the return pipe 38 to bedirectly connected to the main fuel storage tank, designated 20 in thedrawing, in the present invention they are connected instead to avolatising chamber 30 that contains a much smaller quantity of fuel. Thevolatising chamber 30 is connected to the main fuel tank 20 by a supplypipe 24 containing a fuel lifter pump 22 and the level of fuel withinthe chamber 30 is maintained constant by means of a float 28 and a valve26.

An evaporator 40 is disposed in the vapour filled space of the chamber30 above the liquid level and in the path of the fuel returned by way ofthe fuel return pipe 38. The return fuel is sprayed over the evaporatorand the latter is designed to have a large surface area that is coatedwith a film of fuel. The large surface area may be achieved by using amatrix of capillaries or a porous or sintered block for the evaporator40. Neither the evaporator 40 nor the fuel in the chamber 30 is heatedand evaporation relies on the reduced pressure in the vapour space, thedispersion of the spray droplets, the large surface area of theevaporator 40 and such heat as the return fuel picks up during itscirculation flow. The matrix of the evaporator 40 may be formed of ahydrocarbon storage material such as activated carbon to increase thequantity of vapour that can readily be extracted under dynamicconditions.

To maintain the vapour space in the volatising chamber 30 belowatmospheric pressure, a pipe 42 leading from it is connected by way of afirst pipe 46 and a regulating valve 56 to the venturi 12 and by way ofa second pipe 44 and a regulating valve 54 to the intake manifold 16.The pipe 46 is also connected by way of a pipe 48 and a regulating valve58 to a vapour canister 50 that is itself connected to the ullage spaceof the main fuel tank 20 by a pipe 52. Instead of the pipe 48 beingconnected to the pipe 46 to allow fuel vapour stored in the vapourcanister 50 to be purged directly into the venturi 12, it isalternatively possible as represented by the pipe 48′ shown in dottedlines to route the purge flow to the venturi 12 through the volatisingchamber 30.

Under idling and low load conditions, a high vacuum will be present inthe intake manifold 16 which will result in a high rate of evaporationof the fuel in the volatising chamber 30 and the bulk of the fuelrequirement will be delivered to the engine in vapour form. A smallquantity of liquid fuel corresponding to the unvaporised fraction of thefuel will be supplied by the fuel injection system so as to maintain thecomposition of the fuel consumed overall the same as that present in thefuel storage tank 20.

As the engine load is increased progressively, the pressure in theintake manifold 16 will rise towards atmospheric pressure while theventuri pressure will drop with increasing air flow. By suitableselection of the position of the regulating valves 54 and 56 the vacuumpressure in the volatising chamber 30 can be set to supply vapour at anydesired rate while the balance of the fuel to make up the originalcomposition of the fuel is injected by the fuel injectors. During thismode of operation the vacuum alone would not be sufficient to maintainthe rate of vapour supply continuously but as a large proportion of thefuel is recirculated in the loop 32, 34, 36, 38 the cooling of theevaporator 40 will be compensated by heat picked up by the recirculatingfuel and the evaporation rate will stabilise.

The rate of supply of fuel in vapour form to the engine depends upon thepressure and temperature prevailing in the volatising chamber 30 and theposition of the regulating valves 54 and 56. The engine control systemwill first decide the total quantity of fuel to be burnt and thefractions to be supplied in vapour and liquid forms. Based upon thesevariables, as can be prior determined by conventional engine fuelcalibration maps, the engine management system can set the positions ofthe regulating valves 54 and 56 to achieve the desired vapour flow rateand the pulse width of the fuel injectors 18 to achieve the desiredliquid flow rate.

Under high load conditions, there will be hardly any vacuum in theintake manifold 14 but a high vacuum at the venturi 12. However undersuch high load it is not desirable to supply fuel vapour as it wouldreduce the volumetric efficiency and maximum power output of the engine,for which reason the valve 56 can be closed so that all the fuelrequirement is met by the injected liquid fuel.

The fact that fuel vapour is used efficiently in running the engineallows proper use of such vapour as is stored in the vapour canister 50.Whereas normally fuel purged from the canister 50 is merely dumped intothe intake system in an uncontrolled fashion to regenerate the canister50, by routing the purge flow through the volatising chamber 30, suchvapour flow is taken into consideration in determining the total amountof fuel vapour to be metered to the engine.

As discussed above, the invention copes well with a steady demand forfuel vapour as the operating pressure and temperature will moveautomatically to match the demand. To cope with sudden changes in thevapour demand, there is a need for a vapour store to act as a buffer.Such a vapour store is already present in the form of the canister 50the content of which may be used by opening the valve 58 whenever asudden surge occurs in the demand for fuel vapour. A second vapour storecan be formed by using a storage material, such as activated carbon, inthe evaporator 40 which will be replenished more rapidly than the vapourcanister 50.

As well as supplying fuel vapour to the intake system of the engine asdescribed above, a suction pump in the vapour extraction system can beused to supply vapour under positive pressure into the exhaust system ofthe engine. This could be desirable, for example to raise thetemperature of a catalytic convertor either during cold starts or duringlong periods of idling.

The described embodiment uses a matrix as a means for promotingevaporation by increasing the surface area to volume ratio of the returnfuel but other means can be used to achieve the same objective. Forexample, the return fuel may be arranged to pass through a spray nozzleand to be atomised into fine droplets during its entry into thevolatising chamber 30 in order to increase its surface area to volumeratio significantly and thereby promote its vaporisation.

What is claimed is:
 1. A fuel vapour extraction system for an internalcombustion engine (10) supplied with a volatile liquid fuel from a fuelstorage tank (20), the engine (10) having an air intake system(12,14,16) and a liquid fuel injection system for dispensing fuel to mixwith air to be burnt in the engine, the fuel vapour extraction systemincluding a volatising chamber (30) connected to the fuel storage tank(20) by a valve (26) serving to maintain a constant liquid level of fuelin the chamber (30) and a vapour space above the liquid level in thechamber (30), and means (42) for drawing vapour from the vapour space inorder to maintain a reduced pressure in the volatising chamber (30), thefuel injection system including a fuel circulation pump (32) for drawingliquid fuel from the volatising chamber (30) and supplying the fuelunder pressure to a fuel rail (34), fuel injectors (18) for dispensingmetered quantities of fuel from the fuel rail (34) to the enginecylinders, a relief valve (36) for maintaining a constant fuel pressurein the fuel rail (34) and a fuel return pipe (38) for returning unusedfuel from the fuel rail (34) to the volatising chamber (30),characterised by means (40) within the vapour space of the volatisingchamber for promoting evaporation of the return fuel by significantlyincreasing the surface area to volume ratio of the return fuel.
 2. Afuel vapour extraction system as claimed in claim 1, wherein the meansfor drawing the fuel vapour and maintaining a reduced pressure in thevapour space of the volatising chamber comprises a connection (46) byway of a regulating valve (56) to a venturi section (12) in an airintake passage of the engine.
 3. A fuel vapour extraction system asclaimed in claim 1, wherein the means for drawing fuel vapour andmaintaining a reduced pressure in the vapour space of the volatisingchamber comprises a connection (44) by way of a regulating valve (54) toa low pressure region in the intake system of the engine downstream ofthe engine main throttle (14).
 4. A fuel vapour extraction system asclaimed in claim 1, wherein the means for drawing fuel vapour andmaintaining a reduced pressure in the vapour space of the volatisingchamber comprises a vacuum pump driven directly or indirectly by theengine.
 5. A fuel vapour extraction system as claimed in claim 1,wherein the means for promoting evaporation of the return fuel comprisesa matrix of fine capillary tubes or porous granules arranged in the pathof the return fuel.
 6. A fuel vapour extraction system as claimed inclaim 1, wherein the matrix comprises a chemically active material toact additionally as a vapour store.
 7. A fuel vapour extraction systemas claimed in claim 6, wherein the chemically active material includesactivated carbon.
 8. A fuel vapour extraction system as claimed in claim7, wherein the means for promoting evaporation of the return fuelcomprises means for atomising the return fuel into fine droplets.
 9. Afuel vapour extraction system as claimed in claim 2 or claim 3 or anyclaim appended thereto, having means for measuring the pressure andtemperature in the volatising chamber and wherein the engine managementsystem is operative to control the rate of flow of fuel to the engine invapour and liquid forms in dependence upon the prevailing engineoperating conditions and the prevailing pressure and temperature in thevolatising chamber.
 10. A vapour extraction system as claimed in claim1, wherein the fuel storage tank (20) is fitted with a vapour storagecanister (50) and wherein a connection (48, 48′) for purging the vapourcanister leads from the canister to the vapour space of the volatisingchamber (30).